The present invention relates to novel methods of molding crystalline polyolefin resins, in particular, orientated gel molding methods capable of giving, at a high molding rate, molded articles having high rigidity (flexural modulus) and high surface gloss, as well as molded articles obtainable by the methods.
Widely known, practically usable nucleating agents for crystalline polyolefin resins include dibenzylidene sorbitol compounds (hereinafter abbreviated as xe2x80x9cDBS compoundsxe2x80x9d) capable of attaining the highest degree of transparency, which were originally invented in Japan and spread worldwide; shell nucleating agents; and rigidity-imparting nucleating agents such as organic phosphoric acid ester salts, talc and the like. The DBS compounds are of solution type, while the shell nucleating agents and the rigidity-imparting nucleating agents are of dispersion type.
According to the research of Fujiyama et al, when a polypropylene resin composition containing 4-methyl-substituted dibenzylidene sorbitol as a nucleating agent is injection-molded at several resin temperatures in the sol temperature range, i.e., at a resin temperature which are equal to or higher than the sol-gel transition temperature during the heating cycle, the flexural modulus of the obtained molded article increases gradually and linearly with a decrease in the resin temperature. This tendency can be found also with respect to other nucleating agents, such as talc, calcium carbonate and shell nucleating agents. Further, the flexural modulus values of polypropylene resin molded articles prepared using these nucleating agents are similar to one another (Fujiyama et al., J. Appl. Polym. Sci., 42,2739 (1991)). However, no research has been done on molding methods wherein the resin molding temperature is set in the gel temperature range (the temperature range in which the resin is in a gel form).
On the other hand, polyolefins containing a DBS compound as a nucleating agent are usually molded at a temperature higher than the sol-gel transition temperature during the heating cycle, and at the moment, no proposals have been made on molding methods performed paying attention to the presence or absence of network-like crystals.
As a rigidity-imparting nucleating agent, xe2x80x9cNA-11xe2x80x9d (trade name, product of Adeka Argus Chemical Co., Ltd., which is an organic phosphoric acid ester salt, is particularly excellent. However, this nucleating agent causes metal exchange reaction during molding, and sometimes fails to achieve the desired results. Further, unlike solution type nucleating agents, dispersion type nucleating agents are insoluble in resins and difficult to uniformly disperse therein, and therefore they are likely to remain as undispersed matter in the resins. If resins containing such undispersed matter are molded by a method comprising an orientation step, they are liable to be unevenly oriented or broken, resulting in the decrease in the stability of quality or in the yield. Accordingly, the dispersion type nucleating agents are difficult to use because of their narrow application range.
The field of polyolefin resin application is expanding year after year. As the field expands, there are increasing demands, in the fields of automobile parts, electric parts, mechanical parts and other industrial parts, for polyolefin resin molded articles with a still higher rigidity, including those which may be usable as a substitute for engineering plastic resins, as well as for unreactive, high-performance, easily usable nucleating agents of solution type which enable production of such molded articles. However, a high-performance rigidity-imparting nucleating agent which can satisfy such demands has not been found so far. Moreover, no proposals have been made on novel molding methods capable of remarkably improving the rigidity of polyolefin resins by modifying the resin with use of nucleating agents.
An object of the present invention is to provide a novel and useful molding method for imparting excellent rigidity to polyolefin resin molded articles.
In view of the above situations, the present inventors carried out extensive research, focusing on the fact that known nucleating agents, DBS compounds, are self-organized in molten polyolefin resin and form isotropic network-like crystals by sol-gel transition so that the polyolefin resin becomes gel [Kobayashi et al., Journal of the Japanese Rheology Society, 18, 44 (1990)].
Consequently, we developed a novel molding method comprising the step of orientating network-like crystals formed in a molten polyolefin resin in the direction of the resin flow, without dispersing on molecular level (dissolving) the network-like crystals again, i.e., at a temperature in the gel temperature range, to thereby cause crystallization of the polyolefin resin in an orientated manner (this process will be hereinafter referred to as xe2x80x9corientated gel molding methodxe2x80x9d). As a result, we succeeded in remarkably increasing rigidity of polyolefin resin molded articles and attaining a high molding rate and excellent surface gloss. The present invention has been accomplished based on these findings.
The orientated gel molding method, i.e., a novel method of molding a polyolefin resin composition according to the present invention, is characterized by molding a molten polyolefin resin composition containing network-like crystals of at least one dibenzylidene sorbitol compound represented by the formula (1) (namely a composition comprising a molten polyolefin resin and network-like crystals of the dibenzylidene sorbitol compound formed in the molten polyolefin resin): 
wherein R1 and R2 are the same or different and each represent a hydrogen atom, a C1-C4 alkyl group, a C1-C4 alkoxy group or a halogen atom; m and n each represent 1 to 5; p is 0 or 1; when m is 2, the two R1""s may optionally be taken together with the benzene ring to which they are attached to form a tetralin ring; and when n is 2, the two R2""s may optionally be taken together with the benzene ring to which they are attached to form a tetralin ring.
Specifically, the method of the invention is characterized in that it comprises the steps of molding a molten polyolefin resin composition containing network-like crystals of at least one dibenzylidene sorbitol compound (DBS compound) represented by the formula (1) (i.e., a composition comprising a molten polyolefin resin and network-like crystals of the dibenzylidene sorbitol compound formed in the molten polyolefin resin), so as to orientate the network-like crystals in the direction of the flow of the molten resin composition by the shearing force generated by the molding; and cooling the molten polyolefin resin composition to cause the resin to crystallize in an orientated manner.
According to the present invention, the polyolefin crystallizes along the network structure of the dibenzylidene sorbitol compound crystals, so that the polyolefin resin also crystallizes in an orientated manner.